Composition of particulate materials for forming self-lubricating products in sintered steel, product in self-lubricating sintered steel and process for obtaining self-lubricating products in sintered steel

ABSTRACT

The composition includes the iron as the main particulate metallic material; at least one particulate alloy element, with the function of hardening the ferrous structural matrix; and a precursor non-metallic particulate compound, generally a carbide or a carbonate, which is capable of generating, upon its dissociation during the sintering, graphite nodules, whose formation is facilitated: by the precursor compound itself when it includes a chemical element which stabilizes the iron alpha phase of the ferrous structural matrix; or by an additional alloy element included in the composition and which is defined by a chemical element that stabilizes the iron alpha phase during the sintering. The composition can be conformed by compaction or by powder injection molding. The process of the invention leads to obtaining products in self-lubricating sintered steel from said composition.

FIELD OF THE INVENTION

The present invention refers to specific techniques for manufacturingfinished products (pieces) and semi-finished products (severalarticles), conformed from a composition of particulate materials (in theform of metallic and non-metallic powders) and which are designed to besintered, said products comprising, besides the elements constitutive ofthe metallic structural matrix of the product to be formed during thesintering step, a precursor phase of a solid lubricant, in theparticulate form and which, by dissociation during the sintering step,generates precipitates of the solid lubricant in the volume of themetallic matrix, leading to the formation of the micro-structure of aself-lubricating product presenting a continuous metallic matrix andwhich is capable of imparting, to the sintered products, a lowcoefficient of friction allied to high mechanical strength and highhardness of the sintered piece or product. The invention refers to saidmetallurgical composition for forming the self-lubricating material “insitu” during the sintering, to the pieces or products in sintered steelobtained from said composition, as well as to the specific alternativetechniques or processes for obtaining said pieces or products by powdermetallurgy.

BACKGROUND OF THE INVENTION

Given the advanced stage of technological development, there is a needfor developing functional materials with high performance, which arespecifically designed for each particular group of applications. Inseveral mechanical engineering applications, a need exists for materialsthat have, at the same time, high mechanical strength and high wearstrength allied to a low coefficient of friction.

It is estimated that about 35% of the whole mechanical energy producedin the planet is lost by lubrication deficiency and is converted in heatby friction. Apart from the energy loss, the generated heat impairs theperformance of the mechanical system due to the heating. Thus,maintaining a low coefficient of friction in mechanical pieces underfriction is highly important, not only for energy economy, but also toincrease the durability of said pieces and of the mechanical systems inwhich they operate, besides contributing to environment preservation dueto decrease of discarding material.

The way being used to reduce wear and friction between surfaces inrelative movement is to maintain these surfaces separated, interleavinga lubricating layer therebetween. Among possible lubricating ways, thehydrodynamic (fluid lubricants) is the most used. In the hydrodynamiclubrication there is formed an oil film which separates completely thesurface in relative movement. However, it should be pointed out that theuse of fluid lubricants is usually problematic, as in applications atvery high or very low temperatures, in applications in which the fluidlubricant may chemically react and when the fluid lubricant may act as acontaminant. Besides, in situations of limit lubrication resulting fromcycle stops, or in situations in which it is impossible to form acontinuous oil film, there occurs the contact between the pieces,consequently causing wear to the latter.

The dry lubrication, that is, the one using solid lubricants, is analternative to the traditional lubrication, since it acts by thepresence of a solid lubricant layer, which prevents the contact betweenthe component surfaces, but without presenting rupture of the formedlayer.

The solid lubricants have been well accepted in problematic lubricationareas. They can be used in extreme temperatures, under high-loadconditions and in chemically reactive environments, where conventionallubricants cannot be used. Moreover, dry lubrication (solid lubricants)is an environmentally cleaner alternative.

The solid lubricant may be applied to the components of a tribologicalpair, in the form of films (or layers) that are deposited or generatedon the surface of the components or incorporated to the volume of thematerial of said components, in the form of second-phase particles. Whenspecific films or layers are applied and in case they suffer wear, thereoccurs the metal-metal contact and the consequent and rapid wear of theunprotected confronting surfaces and of the relatively movablecomponents. In these solutions in which films or layers are applied, itshould be further considered the difficulty in replacing the lubricant,as well as the oxidation and degradation of the latter.

Thus, a more adequate solution which allows increasing the lifetime ofthe material, that is, of the components, is to incorporate the solidlubricant into the volume of the material constitutive of the component,so as to form the structure of the component in a composite material oflow coefficient of friction. This is possible through the powdermetallurgy techniques, that is, by the conformation of a powder mixtureby compaction, including pressing, rolling, extrusion and injectionmolding, followed by sintering, in order to obtain a continuouscomposite material, usually already in the final geometry and dimensions(finished product) or in geometry and dimensions close to the final ones(semi-finished product).

Self-lubricating mechanical components presenting low coefficient offriction, such as self-lubricating bushings, are produced by powdermetallurgy techniques from metallic powders which form the metallicstructural matrix of the sintered piece and which are mixed withsolid-lubricant powders. Said components have been used in diversehousehold appliances and small equipment, such as: printers, electricshavers, drills, blenders, and the like. Most of the well-known priorart solutions for the structural matrix use bronze, copper, silver, andpure iron. There are used as solid lubricant: molybdenum disulfide(MoS₂), silver (Ag), polytetrafluoroethylene (PTFE) and molybdenumdiselenide (MoSe₂). Bushings with these types of self-lubricatingmaterials, mainly with bronze and copper matrix containing, such assolid lubricant particles, graphite powder, selenium and molybdenumdisulfide and low melting point metals, have been produced and used fordecades in several engineering applications.

However, these pieces do not present high mechanical strength, as afunction of its high volumetric content (from 25% to 40%) of solidlubricant particles, which results in a low degree of continuity of thematrix phase, which is the micro-structural element responsible for themechanical strength of the piece. This high content of solid lubricanthas been considered necessary for obtaining a low coefficient offriction in a situation in which both the mechanical properties of themetallic matrix (strength and hardness) and the micro-structureparameters, such as the size of the solid lubricant particles dispersedin the matrix and the average free path between these particles in theformed composite material, are not optimized for applications in whichthe piece is required to have high mechanical strength. The highvolumetric percentage of solid lubricant, which has an intrinsic lowstrength to shearing, does not contribute to the mechanical strength ofthe metallic matrix. Furthermore, the solid lubricant particles sheareasily and alter their shape, as a function of shearing forces thatoccur during the steps of mechanically homogenizing the powder mixture(carried out in mixers) and compacting the mixture, reducing even morethe degree of continuity of the metallic structural matrix of the formedself-lubricating composite. Moreover, the low hardness of the metallicmatrix allows a gradual obstruction of the solid lubricant particles tooccur on the contact surface of the sintered material or product. Thus,in order to maintain a sufficiently low coefficient of friction, therehas been traditionally used a high volumetric percentage of solidlubricant in the composition of dry self-lubricating compositematerials.

A partially differentiated and more developed scenario, as compared withthat previously described, is disclosed in U.S. Pat. No. 6,890,368A,which proposes a self-lubricating composite material to be used attemperatures in the range between 300° C. and 600° C., with a sufficienttraction resistance (R_(m)≧400 MPa) and a coefficient of friction lowerthan 0.3. This document presents a solution for obtaining pieces orproducts of low coefficient of friction, sintered from a mixture ofparticulate material which forms a metallic structural matrix andincluding, as solid lubricant particles in its volume, mainly hexagonalboron nitride, graphite or a mixture thereof, and states that saidmaterial is adequate to be used at temperatures in the range between300° C. and 600° C., with a sufficient traction resistance (R_(m)≧400MPa) and a coefficient of friction smaller than 0.3.

As described in the Brazilian patent application (provisional number018080057518) filed on Sep. 12, 2008, in the name of the same applicantsof the present invention, pieces or products obtained from theconsolidation of a powder mixture simultaneously presenting thestructural matrix powders and the solid lubricant powders, such as forexample, hexagonal boron nitride and graphite, have low mechanicalstrength and structural fragility after sintering.

The deficiency cited above results from the inadequate spread(dispersion), by shearing, of the solid-lubricant phase between thepowder particles of the structural matrix during the steps of mixing andconforming (densification) the pieces or products to be produced. Thesolid lubricant spreads, by shearing, between the particles of thestructural matrix phase, and tends to surround said particles during themixing and conforming steps, which submit said solid lubricant tostresses which surpass its low shearing stress.

On the other hand, the presence of the solid lubricant layer between theparticles (of the powder) of the structural matrix, formed by shearing,impairs the formation of metallic contacts between these particles whichform the structural matrix of the composite during the sintering; thiscontributes to a reduction of the degree of continuity of the structuralmatrix phase of the composite material, structurally fragilizing thematerial and the obtained products.

Such problems can be mostly solved through solutions proposed in theprior Brazilian patent application mentioned above, resulting inobtaining composite materials with mechanical strength greater than thatof the prior art solutions.

However, in the solution proposed in said prior patent application ofthe same applicants of the present invention, the non-metallicparticulate solid lubricant, for example hexagonal boron nitride,graphite or both, has to be mixed to the metallic materials which formthe structural matrix of the composite product to be sintered, furtherrequiring the addition of at least one particulate alloy element, so asto form, during the sintering step of the conformed metallurgicalcomposition, a liquid phase between the particulate material which formsthe structural matrix and the non-metallic particulate solid lubricant,in order to agglomerate the latter in discrete particles and prevent thenon-metallic particulate solid lubricant from spreading, by shearing,between the particles of the structural matrix phase, tending tosurround them during the steps of mixing and conforming (densification)the pieces or products to be produced, fragilizing the latter.

In face of the drawbacks cited above, it is desirable to provide asolution which does not require the previous mixing of the solidlubricant particles in the metallurgical composition to be sintered, northe addition of an alloy element in the metallurgical composition toform a liquid phase in the latter during its sintering.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide acomposition of particulate materials for forming sintered steels,comprising a metallic structural matrix which permits, per se and duringsintering thereof, forming a finished or semi-finished product inself-lubricating sintered steels with a high degree of continuity of thestructural matrix and presenting high mechanical strength and highhardness, with a fine distribution of a solid lubricant phase generatedin the sintering.

It is likewise an object of the present invention to provide a productin self-lubricating sintered steel, obtained from a conformation bypowder compaction via pressing, rolling and others or by injectionmolding, followed by sintering the composition defined above, and whichpresents a high degree of continuity of the metallic structural matrix,a low coefficient of friction and high mechanical strength and highhardness, with a fine distribution of a solid lubricant phase ofgraphite generated in the sintering.

It is also another object of the present invention to provide a processfor obtaining products in self-lubricating sintered steel, such asdefined above, from said composition of particulate materials, saidprocess including neither the previous mixture of solid lubricantparticles in the metallurgical composition to be sintered, nor theaddition of an alloy element in the metallurgical composition to form aliquid phase in the latter during its sintering.

In a first aspect of the present invention, the objects cited above areattained through a composition of particulate materials for themanufacture of products in self-lubricating sintered steel, previouslyconformed by one of the operations of compacting and injection moldingsaid composition which comprises: the iron as the main particulatemetallic material; at least one particulate alloy element, with thefunction of hardening the iron, forming therewith a ferrous structuralmatrix; and a non-metallic compound, precursor of a solid lubricantphase of graphite to be formed in the product during the sintering.

In a way of carrying out the invention, the non-metallic particulatecompound is a compound of the carbide or carbonate type including achemical element which stabilizes the iron alpha phase of the ferrousstructural matrix. In another way of carrying out the present invention,the non-metallic particulate compound is deprived of any chemicalelement which stabilizes the iron alpha phase, thus being necessary toinclude, in the metallurgical composition, an additional particulatealloy element which has the function of stabilizing the iron alphaphase.

In the present invention, there occurs the formation of graphiteparticles by dissociation of a precursor phase during the sintering stepof the pieces or products. As examples of precursor phases for carryingout the invention, it can be cited: silicon carbide (SiC), molybdenumcarbide (Mo₂C), chromium carbide (Cr₃C₂), and the like. In the step ofpreparing the powder mixture which will constitute the new compositematerial, carbides in the form of fine powder particles (preferably from5 to 25 μm) are mixed to the iron powder (major component) and otherpowders of alloy elements that are present in the powder mixture. Themost indicated carbides to cause precipitation of graphite nodules inferrous matrix, forming a self-lubricating sintered steel, are thosewhich have in their formula a chemical element which can stronglystabilize the iron alpha phase, as for example, the element Si presentin the silicon carbide (SiC). During the sintering step, that is, at thesintering temperature of the pieces or products, the silicon carbide(SiC) dissociates and the chemical element silicon becomes a solidsolution in the iron, that is, in the ferrous structural matrix. As thedissociation of the SiC progresses, the amount of solubilized Siincreases in the ferrous matrix in the surroundings of the SiC particleswhich are in dissociation. As can be verified in the iron-siliconequilibrium diagram, the chemical element silicon strongly stabilizesthe iron alpha phase; the vertex of the loop α⇄ (α+γ) in Fe—Si diagramoccurs for values of 2.15% by weight (4.2% at) of Si. Thus, during thesintering of the sintered steel, carried out typically between 1125° C.and 1250° C., when the concentration of silicon solubilized in iron,around the SiC particle in dissociation, reaches the solubility limit ofthe gamma-phase, there occurs a transformation of the gamma-iron intoalpha-iron. In the first instants of the SiC dissociation process, whilethe Si concentration does not reach the value required to stabilize thealpha-phase around the SiC particle in dissociation, the carbonresulting from the dissociation also becomes a solid solution andspreads to the interior of the matrix, but as soon as the ferrous matrixaround the SiC particulate in dissociation is transformed intoalpha-phase, the process for solubilizing carbon is interrupted becausethe solubility of carbon in the iron alpha phase is very low (maximumvalue is of 0.022% by weight at 727° C.). Thus, the carbon released, asa consequence of the carbide dissociation, forms graphite nodules, whichare surrounded by a layer of alpha-iron, although the remainder of thematrix can continue presenting the gamma phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below, with reference to the encloseddrawings, given by way of example of embodiments of the invention and inwhich:

FIGS. 1A, 1B and 1C represent, sequentially and schematically, theevolution of the micro-structure during the sintering step, resultingfrom the dissociation of the carbide particles mixed with the ironpowder (matrix), FIG. 1A representing the two-phase micro-structure ofthe material in the initial phase of the process, in which the carbideparticles are still intact, that is, the reaction has not yet initiated,whilst FIG. 1B represents the situation in which there has alreadyoccurred partial dissociation of the carbides and FIG. 1C shows thesituation in which the dissociation has already been completed;

FIG. 2 shows, schematically, the desired ideal situation(microstructural model) for the distribution of the solid lubricantparticles or nodules in the volume of a composite material, in steel,with low coefficient of friction, allowing maintaining a high degree ofcontinuity of the matrix of the composite material; in an idealsituation, the solid lubricant must be in the form of discrete particlesor nodules uniformly distributed in the volume of the material, with aregular average free path “λ” between the particles or nodules;

FIG. 3 is a picture of the micro-structure of the material of thepresent invention in the already sintered state, after the dissociationof the carbide particles, showing the graphite nodules surrounded by aclear layer which is formed by the alpha-phase, and the matrix of thecomposite material;

FIG. 4 shows a detail of the graphite structure in the interior of thenodule generated during the sintering, through a picture obtained with ahigh increase (of 20,000×) in the scanning electron microscope withfield emission gun (FEG-SEM), which evidences the structure in the formof graphite skins or flakes of nanometric thickness;

FIG. 5 represents, schematically and in a simplified diagram, an exampleof compaction in the formation of a piece or product to be posteriorlysintered, said compaction being made so as to provide a self-lubricatinglayer in two opposite faces of the product to be sintered; this processshould be used when it is desired only one self-lubricating layer in oneor more faces of the sintered piece;

FIGS. 6A, 6B and 6C represent examples of products whose conformation isobtained by compaction carried out by extrusion, respectively, of a barin a self-lubricating composite material, of a tube in aself-lubricating composite material, and of a bar with a core inmetallic alloy coated with an outer layer with a self-lubricatingmaterial; and

FIG. 7 represents, schematically and in a simplified diagram, an exampleof compaction in the formation of a piece or product to be posteriorlysintered, said compaction being made by rolling a self-lubricatingcomposite material on the opposite faces of a plate or strip in metallicalloy.

DESCRIPTION OF THE INVENTION

As already previously mentioned, one of the objects of the invention isto provide a composition of particulate materials, which can behomogeneously mixed and conformed (densified) by compaction (pressing,rolling) or by extrusion or injection molding of powders, so that it mayassume a defined geometry (piece) to be submitted to a sinteringoperation, in order to obtain a product which presents high hardness,mechanical strength and reduced coefficient of friction in relation tothe products obtained by the prior art teachings. The presentcomposition comprises: a main particulate metallic material which ispreponderant in the formation of the composition, and at least oneparticulate alloy element with the function of hardening thepreponderant material, these components being responsible for theformation of a structural matrix in the composite product, in steel, tobe sintered; and a precursor particulate material which allows obtainingsolid lubricant nodules upon its dissociation during the sintering.

According to the invention and as illustrated in FIG. 2, the mainparticulate metallic material is iron, defining a ferrous structuralmatrix 10, and the precursor phases for generation of nodules 20 ofsolid lubricant by dissociation during the sintering are compounds basedon carbides or carbonates, preferably formed with chemical elementswhich stabilize the iron alpha phase in the ferrous structural matrix10. When the precursor phase used does not have, in its composition, achemical element capable of stabilizing the iron alpha phase in theferrous matrix 10, a specific additional alloy element in a sufficientamount to stabilize the iron alpha phase should also be added to thecomposition of the material of the present invention.

The alloy element with the function of hardening the ferrous structuralmatrix is defined, for example, by one of the elements selected fromchrome, molybdenum, carbon, silicon, phosphorus, manganese and nickel,but it should be understood that one can use other elements, such asvanadium and copper, which carry out the same function in the structuralmatrix, as well as more than one alloy element at the same time. Itshould be noted that the invention requires the provision of an alloyhardening element which may carry out the function of hardening theferrous structural matrix to be formed during the sintering, byinterdiffusion of the components (chemical homogenization), but thisaspect should not be limited to the alloy elements exemplified herein.

FIGS. 1A, 1B, 1C and 2 show, schematically, several steps of theevolution of the micro-structure of the composite as a function of thedissociation of the carbide (SiC) during the sintering. FIG. 3 shows apicture, obtained by optical microscopy, of the micro-structure of thecomposite material formed after its sintering, and FIG. 4 shows thestructure of the precipitate graphite presenting, in the interior of thenodules, the form of “skins or leaves” of nanometric thickness. Thisstructure favors the formation of a tribological layer on the interfaceof the relative moving surfaces of the tribological pair, increasing theefficiency of the solid lubrication.

The parallel addition of other alloy elements, which stabilize the ironalpha phase in the powder mixture which will form the composite,accelerates the rising of the alpha phase in the matrix during thesintering operation, increases the tendency to generate graphite nodules20 by dissociation of carbide particles mixed in the volume of thematerial.

The alloy elements which stabilize the iron alpha phase and coming fromthe carbide dissociation, besides preventing the carbon fromsolubilizing in the matrix, since they form a layer of alpha phase 12around the particle 11 in dissociation, they also contribute to increasethe hardness of the matrix when in solid solution; nevertheless, if thehardness increase reached by the presence of these alloy elements insolid solution in the iron is not sufficient, other alloy elementsshould be additionally added to the powder mixture, so as to besolubilized in the matrix during the sintering operation, aiming atachieving the hardness and mechanical strength necessary for theapplication.

Thus, in the present invention, the metallic structural matrix of thematerial is formed by iron automatically hardened by a solid solutionwith the alloy elements which stabilize the iron alpha phase, as forexample, silicon and molybdenum dissolved in the ferrous matrix as aconsequence of the dissociation of the carbides mixed to the iron powderin the processing of the material by powder metallurgy.

Besides these necessarily present stabilizing alloy elements, otheralloy elements might be added to the powder mixture with the function ofadjusting the mechanical strength and the hardness of the matrix,allowing reaching a high performance in relation to the tribological andmechanical behavior of the dry self-lubricating composite materialgenerated during the sintering. As examples of other alloy elements,advantageously used in the present invention, to increase the mechanicalstrength and the hardness of the matrix, besides the Si, Mo, and Pelements, which are strong stabilizers of the iron alpha phase, therecan be cited the elements Cr, Ni, Mn, W, V, and C.

As to the types of carbides used, the powder mixture composition, whichis formulated for the production of products by powder metallurgy in thepresent invention, is formed by two distinct alternatives:

Alternative 1: Iron powder+particles 11 of carbide powder which areformed by chemical elements which stabilize the iron alpha phase (mixedin a volumetric percentage ≦10%), which, at the sintering temperature,generate graphite nodules 20 upon dissociation thereof, +powderparticles of other chemical elements called alloy elements, which havethe function of increasing the hardness and the strength of the ferrousstructural matrix 10;

Alternative 2: Iron powder+carbide powder particles which are not formedby chemical elements which stabilize the iron alpha phase (mixed in avolumetric percentage ≦10%), +powder of alloy elements which stabilizethe iron alpha phase which has the function of stabilizing the alphaphase of the ferrous matrix, in order to prevent the carbon coming fromthe carbide dissociation from being dissolved by the ferrous matrix,+other alloy elements which are present for adjusting the mechanicalproperties of the structural matrix of the composite. Since the metallicferrous structural matrix 10 is the sole micro-structural element of thecomposition that confers mechanical strength to the composite materialto be formed, the higher the degree of continuity of the matrix of saidcomposite, the higher will be the mechanical strength of the sinteredarticle or piece produced with the material. The maintenance of the highdegree of continuity of the metallic structural matrix of the dryself-lubricating sintered composite material requires, besides a lowporosity, a low volumetric percentage of the solid lubricant phase,since the latter does not contribute to the mechanical strength of thematerial and, consequently, does not contribute to the mechanicalstrength of the sintered products. Besides, the solid lubricant which ispresent in the volume of the material should be dispersed in the form ofdiscrete particles or nodules 20, uniformly distributed in the volume,that is, with a regular average free path “λ” in the interior of theferrous structural matrix 10 (see FIG. 2). This permits generatinggreater lubrication efficiency and, at the same time, guarantees ahigher degree of continuity of the matrix, which on its turn guaranteesa higher mechanical strength to the composite material.

The metallic matrix of the material is required to be highly resistantto plastic deformation, in order to operate not only as a mechanicalsupport with the necessary load capacity, but also to prevent the solidlubricant particles from being covered by plastic deformation of thestructural matrix, upon operation of the piece (when frictioned inrelative movement), preventing the solid lubricant from spreading in theinterface where it should form a layer of solid lubricant.

According to the invention, the additional alloy component, whichstabilizes the iron alpha phase, is defined by at least one of theelements selected from phosphorus, silicon, cobalt, chrome andmolybdenum. Although these elements are considered the most adequate toseparately or jointly act in stabilizing the iron alpha phase atsintering temperatures (about 1125° C. to about 1250° C.), it should beunderstood that the invention resides in the concept of stabilizing theiron alpha phase, in order to impair the carbon dissolution, and not inthe fact that the alloy component(s) used are necessarily the onesexemplified herein.

When the composition of the invention is conformed by compaction, themain particulate metallic material (iron) presents, preferably, anaverage particle size lying between about 5 μm and about 90 μm. On itsturn, the hardening element, with the function of hardening thestructural matrix, and the precursor component of the solid lubricantphase (compound) should present a particle size preferably smaller thanabout de 45 μm; it should be further understood that the averageparticle size of the main particulate metallic material, that is, of theiron, should be always larger than the average particle size of thealloy elements and the precursor components (compounds) of the solidlubricant phase.

When the composition of the invention is conformed by injection molding,the main particulate metallic material (iron) presents, preferably, aparticle size lying between about 5 μm and about 25 μm. In the same way,the alloy elements and the precursor components (compounds) of the solidlubricant phase present, preferably, a particle size also between about5 μm and about 25 μm.

When the conformation of the composition, previous to the sintering, iscarried out by extrusion or by injection molding, the composition shouldfurther comprise at least one organic binder selected preferably fromthe group consisting of paraffin and other waxes, EVA, and low meltingpoint polymers in a proportion generally ranging from about 15% to about45% of the total volume of the metallurgical composition, upon theconformation by extrusion, and from about 40% to 45%, upon theconformation by injection molding. The organic binder is extracted fromthe composition after the conformation step, for example by evaporation,before the conformed product is conducted to the sintering step.

The compositions described above are obtained by mixing, in any adequatemixers, predetermined quantities of the particulate materials selectedfor the formation of the composition and for the subsequent obtention ofa self-lubricating sintered product.

The mixture of the different particulate materials is homogenized andsubmitted to a densification operation by compaction, that is, bypressing or rolling, or also by molding by extrusion or injection ofpowders, obtaining in this operation, not only the densification of thepowder mass, but also the desired shape for the product to be obtainedby sintering.

In case of conformation by powder molding by extrusion or injection, themixture of the components containing the organic binder is homogenizedat temperatures not inferior to the melting temperature of the organicbinder, the thus homogenized mixture being granulated to facilitate itshandling, storage and supply to an injection machine.

After conformation of the piece, this is submitted to the extraction ofthe organic binders, generally carried out in two steps, the first stepbeing a chemical extraction process in solvents (for example, hexane)and the second step being an extraction process by thermal degradation,or a CD plasma assisted thermal process.

With the composition proposed herein, it is possible to obtainself-lubricating sintered pieces or products with hardness from 230 HVto 700 HV, a coefficient of friction μ≦0.15, a mechanical tractionresistance from 350 to 750 MPa (depending on the alloy elements whichare present and on the processing parameters used) and also with adispersion of amorphous carbon nodules with the inner structure in theform of skins with nanometric thickness, which facilitates the spreadingof the graphite in the interface of the movable surfaces, forming asolid lubricant layer.

FIGS. 5, 6A, 6B, 6C and 7 of the enclosed drawings have the purpose ofexemplifying different possibilities of conforming the presentcomposition, by compacting a certain predetermined quantity of thecomposition to any desired shape, which can be that of theself-lubricating sintered final piece or product desired to be obtained,or a shape close to that desired final one.

However, in a large number of applications, the self-lubricatingcharacteristic is necessary only in one or more surface regions of amechanical component or piece to be submitted to a friction contact withother relatively movable element.

Thus, the desired self-lubricating product can be constituted, asillustrated in FIG. 5, by a structural substrate 30 preferably conformedin a particulate material and receiving, in one or two opposite faces31, a surface layer 41 of the composition 40 of the present invention.In the illustrated example, the structural substrate 30 and the twoopposite surface layers of the composition 40 are compacted in theinterior of any adequate mold M, by two opposite punches P, forming acompacted and conformed composite product 1, which is posteriorlysubmitted to a sintering step. In this example, only the two oppositefaces 31 of the structural substrate 30 will present the desirableself-lubricating properties.

FIGS. 6A and 6B exemplify products in the form of a bar 2 and a tube 3,respectively, obtained by extrusion of the composition 40 in an adequateextrusion matrix (not illustrated). In this case, the conformation bycompaction of the composition 40 is carried out in the extrusion step ofthe latter. The bar 2 or tube 3 can then be submitted to the sinteringstep, for the formation of the iron-based structural matrix 10 andincorporating discrete dispersed and particles of the particulate solidlubricant 20.

FIG. 6C illustrates another example of product formed by a composite bar4, comprising a structural core 35, in a particulate material and whichis circumferentially and externally surrounded by a surface layer 41formed from the composition 40 of the invention. Likewise in this case,the conformation and the compaction (densification) of the structuralcore 35 and of the outer layer 41 in the composition 40 are obtained byco-extrusion of the two parts of the composite bar 4, which is thensubmitted to the sintering step.

When the compaction of the composition 40 is carried out by extrusion,as it occurs, for example, in the formation of the bars 2, 3 and 4 ofFIGS. 6A, 6B and 6C, said composition can further comprise an organicbinder which is thermally removed from the composition, after theconformation of the latter and before the sintering step, by any of theknown techniques for said removal.

The organic binder may be, for example, any one selected from the groupconsisting of paraffin and other waxes, EVA, and low melting pointpolymers.

FIG. 7 represents, also schematically, another way to obtain a compositeproduct in sintered steel presenting one or more surface regions havingself-lubricating characteristics. In this example, the product 5 to beobtained presents a structural substrate 30 formed in a particulatematerial, previously conformed in the form of a strip, it being notedthat, on at least one of the opposite faces of the structural substrate30, in a continuous strip, there is rolled a surface layer 41 of thecomposition 40 of the present invention. The composite product 5 is thensubmitted to a sintering step.

While the invention has been presented herein by means of some examplesof possible compositions and associations with different structuralsubstrates, it should be understood that such compositions andassociations can suffer alterations that will become evident to thoseskilled in the art, without departing from the inventive concept ofcontrolling the distribution, in discrete particles, of the solidlubricant in the structural matrix, and also of the eventual tendency ofsaid solid lubricant to dissolve in said matrix, during the sinteringstep, as defined in the claims that accompany the present specification.

1. A composition of particulate materials for forming self-lubricatingproducts in sintered steel, conformed by compaction or powder injection,characterized in that it comprises: the iron as a main particulatemetallic material; at least one particulate alloy element, with thefunction of hardening the iron, forming therewith a ferrous structuralmatrix; and a non-metallic compound, precursor of a solid lubricantphase of graphite to be formed in the composite product during thesintering.
 2. The composition, as set forth in claim 1, characterized inthat the non-metallic particulate compound, precursor of the solidlubricant phase of graphite, is a compound of the carbide or carbonatetype and which includes, in its composition, a chemical element whichstabilizes the iron alpha phase of the ferrous structural matrix.
 3. Thecomposition, as set forth in claim 2, characterized in that the chemicalelement which stabilizes the iron alpha phase is selected betweensilicon carbide, molybdenum carbide and chromium carbide.
 4. Thecomposition, as set forth in claim 1, characterized in that it furtherincludes an additional particulate alloy element, which stabilizes theiron alpha phase when the non-metallic particulate compound is a carbideor carbonate deprived, in its composition, of any chemical element whichstabilizes the iron alpha-phase of the ferrous matrix.
 5. Thecomposition, as set forth in claim 4, characterized in that theadditional particulate alloy element, which stabilizes the iron alphaphase of the ferrous structural matrix, is at least one the elementsselected from silicon, phosphorus, molybdenum and chrome.
 6. Thecomposition, as set forth in claim 2, characterized in that thenon-metallic particulate compound, precursor of the solid lubricantphase of graphite, represents, preferably, a volumetric percentage lowerthan about 10% of the mass of the particulate material metallurgicalcomposition to be conformed.
 7. The composition, as set forth in claim1, characterized in that the particulate alloy element, with thefunction of hardening the iron of the ferrous structural matrix isdefined by at least one of the elements selected from nickel, chrome,molybdenum, vanadium, manganese, copper, silicon, phosphorus and carbon.8. The composition, as set forth in claim 1, said composition beingconformed by powder compaction (pressing, rolling, double pressing orcompaction) and characterized in that the particles of the mainparticulate metallic material (iron powder) presents an average sizelying between about 5 μm and about 90 μm, the particles of theparticulate alloy element with the function of hardening the iron, andthe particles of the non-metallic particulate compound which is theprecursor of the solid lubricant phase present a size smaller than about45 μm.
 9. The composition, as set forth in claim 8, characterized inthat the average particle size of the main particulate metallicmaterial, that is, of the iron, is larger than the average particle sizeof the particulate alloy element and of the non-metallic particulatecompound, precursor of the solid lubricant phase.
 10. The composition,as set forth in claim 1, the composition being conformed by extrusion orby injection molding and characterized in that the main particulatemetallic material in iron, as well as the particulate alloy element andthe non-metallic particulate compound present a particle size lyingbetween about 5 μm and about 25 μm.
 11. The composition, as set forth inclaim 10, characterized in that it comprises a system of organic bindersselected from the group consisting of paraffin and other waxes, EVA, andlow melting point polymers, in a proportion ranging from about 40% toabout 45% of the total volume of the metallurgical composition.
 12. Aproduct in self-lubricating sintered steel, obtained from a compositionof particulate materials, as defined in claim 1 and submitted to aconformation previous to the sintering, characterized in that itpresents a hardness between 230 HV and 700 HV, a coefficient of frictionμ≦0.15 and a traction resistance between 350 and 900 MPa.
 13. Theproduct, as set forth in claim 12, characterized in that it defines atleast one surface layer of said metallurgical composition incorporatedto a structural substrate.
 14. The product, as set forth in claim 13,characterized in that the structural substrate is defined in aparticulate material to be sintered jointly with the surface layer ofthe metallurgical composition.
 15. The product, as set forth in claim14, characterized in that the structural substrate takes the form ofplate of strip with at least one of its opposite faces incorporating asurface layer of said metallurgical composition.
 16. The product, as setforth in claim 14, characterized in that the structural substrate takesthe form of the structural core of a composite bar, circumferentiallyand externally incorporating a surface layer of said metallurgicalcomposition.
 17. A process for obtaining self-lubricating products insintered steel, from the composition of particulate materials as definedin claim 8, characterized in that it comprises the steps of: mixing, inpredetermined quantities, the particulate materials which define themetallurgical composition; homogenizing the particulate materialmixture; compacting the particulate material mixture, so as to providethe mixture with the shape of the product to be sintered; sintering thecompacted and conformed mixture, at temperatures from about 1125° C. toabout 1250° C., forming, during the sintering, graphite nodules by thedissociation of the precursor compound in the volume of the structuralmatrix.
 18. The process, as set forth in claim 17, characterized in thatthe step of compacting the particulate material mixture, which definesthe composition, comprises rolling the latter in a plate or strip to besubsequently sintered.
 19. The process, as set forth in claim 17,characterized in that the step of compacting the particulate materialmixture, which defines the composition, comprises rolling the latter onat least one of the opposite faces of a structural substrate in the formof a plate or strip of particulate material compatible with the mainparticulate metallic material which forms the structural matrix.
 20. Theprocess, as set forth in claim 18, characterized in that it comprises,after the sintering of the particulate materials, the additional step ofcold rolling the plate or strip for reducing the residual porosity,followed by an eventual annealing.
 21. The process, as set forth inclaim 18, characterized in that the step of compacting the particulatematerial mixture, which defines the composition, comprises the extrusionin one of the shapes defined by a bar and a tube.
 22. The process, asset forth in claim 17, characterized in that the step of compacting theparticulate material mixture, which defines the composition, comprisesthe extrusion of the latter in the form of a tubular sleeve around astructural core in the form of a bar of particulate material compatiblewith the main particulate metallic material which forms the structuralmatrix, so as to form a composite bar.
 23. The process, as set forth inclaim 21, characterized in that the composition comprises an organicbinder to be thermally removed from the product, before the sinteringstep.
 24. A process for obtaining self-lubricating products in sinteredsteel, from the composition of particulate materials, as defined inclaim 10, and characterized in that it comprises the steps of: mixing,in predetermined quantities, the particulate materials which define themetallurgical composition; homogenizing the particulate materialmixture, at a temperature not inferior to that of melting the organicbinder; granulating the composition to facilitate its handling, storageand supply into an injection machine; injection molding the particulatematerial mixture, so as to provide the mixture with the shape of theproduct to be sintered; extracting the organic binder from the moldedpiece; and sintering the pieces obtained by conformation of the powders,at temperatures from about 1125° C. and about 1250° C.